Single laser exposure and monitoring of holograms

ABSTRACT

A system in which the light of a single laser is used to both expose a hologram and to simultaneously measure the formation of the hologram in the recording material.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used, or licensed byor for the Government for Governmental purposes without the payment tome of any royalties thereon.

BACKGROUND OF THE INVENTION

In the Past, various known techniques for the measurement of hologramformation has been used, but these techniques are not suitable fornormal silver-halide based photographic films since light forms a latentimage in these films that is not revealed until development. However,photopolymer films and other recording media begin to form a hologramalmost immediately upon exposure and consequently, it is possible torecord a hologram with a laser of one color and to view its formationsimultaneously with a second laser of another color. The color of thesecond laser or probe, is chosen so as to have no physical or chemicaleffect upon the recording medium. The two laser technique forphotopolymer holograms is discussed in prior art publications. Similartechniques are also used in chemistry in which a holographic exposureilluminates a mixture, and the formation of a transient or permanenthologram gives a measure of chemical reactions. The two-laser techniquecan also be applied here as well as a single laser technique. Thesechemical applications are discussed in the prior art publications also.

Even in light of the prior art there is a need for a device that can beused in the study and characterization of holographic recordingmaterials and films that utilize a single laser source.

Therefore, it is an object of this invention to provide a device thatutilizes a single laser source in the study and characterization ofholographic recording materials and films and a device that can be usedin the control of holographic exposure systems to cause an optimumexposure to be produced.

Other advantages and objects of this invention will be obvious to thosewho are skilled in this art.

SUMMARY OF THE INVENTION

In accordance with this invention, a system for exposing a holographicrecording medium and for simultaneously measuring the formation of thehologram in the recording medium is provided and includes a single lasersource that is split by a beam splitter into an object beam and areference beam that are used to expose a holographic recording medium.Shutters are placed in the paths of the object beam and the referencebeam before the holographic recording medium and the opening and closingof the shutters are controlled as desired depending upon the exposuredesired of the holographic recording medium. A photodiode is mountedbeyond the holographic recording medium and in the path of the referencebeam. The photodiode is utilized to measure the formation of thehologram with the detected formation of the hologram being used in dataacquisition to produce signals that can be used with the shutter controlfor control of the shutters in the required or desired development ofthe hologram.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the system in accordance with thisinvention, and

FIG. 2 is an illustration of the position of the shutters in the threedifferent modes of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a laser source 10 is utilized toilluminate a beam splitter 12 which splits the beam into an object beam14 and a reference beam 16. Mirrors 18 and 20 or other suitable opticaldevices are placed so as to cause beams 14 and 16 to be deflected andmeet at a front surface of holographic recording medium 22. Beams 14 and16 jointly form an interference pattern and a record of thisinterference pattern impressed on recording medium 22 produces ahologram. The arrangement described thus far is known most commonly asthe Leith-Upatnieks type of holograms. To this arrangement shutter 24 isplaced in the path of object beam 14 and shutter 26 is placed in thepath of reference beam 16 as illustrated. Shutters 24 and 26 are placedin the beam paths such that they can block the light of each beam beforeit impinges on holographic recording material 22. Shutters 24 and 26 canbe electro-mechanical vane shutters, rotating beam choppers,electro-optic shutters, or any other device that can be used toalternately block and pass light. Object beam shutter 24 and referencebeam shutter 26 are controlled by shutter control 28 through connectingmeans 30 and 32 to open and close shutters 24 and 26 as desired forexposing holographic recording medium 22. Shutter control device 28 isused to issue the desired signals to cause shutters 24 and 26 to openand close at specific times. A photodetector 34 of conventional typestructure is placed in the path of reference beam 16 beyond holographicrecording material 22 as illustrated. A data acquisition device 36 issynchronized with shutter control device 28 to measure through link 38the light incident on photodetector 34 at specific times, and to storethis data. Photodetector 34 is connected by link 38 to data acquisitiondevice 36 in a conventional manner. Shutter control device 28 and dataacquisition device 36 can be utilized as general purpose computers,special purpose computers, or other logic devices with appropriateclocks and analog to digital converters. Also, if desired shuttercontrol device 28 and data acquisition device 36 can be incorporatedinto one computer or control device.

In operation, the operation cycle is based on the fact that when ahologram is present in hologram recording medium 22, and the hologram isilluminated by the object beam 14, some portion of the object beam lightis diffracted by the hologram to form a replica of the originalreference beam. The diffracted light travels from the hologram in thepath of the reference beam and falls upon photodetector 34. Thediffracted light is measured and stored in data acquisition device 36.The fraction of the object beam light that is diffracted into thereference beam is called the diffraction efficiency. This efficiency isan important measure of the quality of the hologram.

Applicant's device has three different specific modes of operation thatare identified as exposure mode, dark mode, and flood light mode. Theshutter actions for each of these modes is shown on the attached shuttertime diagram as illustrated in FIG. 2. The specific mode used dependsupon the nature of the particular holographic recording medium used.Exposure is necessary in all cases, and various recording media call forprocessing by heat, chemicals, or other means in the dark. Some mediarequire post exposure illumination by a bright, uniform flood light.

In the exposure mode, object beam shutter 24 is in the open positioncontinuously and reference beam shutter 26 Is open most of the time asillustrated in FIG. 2. At predetermined regular intervals, referencebeam shutter 26 is closed briefly for a short time and then reopened asillustrated in FIG. 2. During the brief closure of reference beamshutter 26, the only light that reaches photodetector 34 is the lightdiffracted from object beam 14. Data acquisition device 36 issynchronized with shutter control 28 to know when shutter 26 is closedso that the intensity of the diffracted light can be measured and storedin data acquisition device 36. The total fraction of time that referencebeam shutter 26 is closed during exposure (i.e., the duty cycle) shouldbe small in comparison to the on time and the length of each closure ofshutter 26 should be small relative to the reaction time of holographicrecording medium 22.

In the dark mode, reference beam shutter 26 is closed continuously.Object beam shutter 24 is opened briefly at regular intervals asillustrated in FIG. 2 and data acquisition device 36 is synchronizedwith shutter 24 to measure the light diffracted from object beam 24. Theduty cycle, when the object shutter 24 is opened, must be small incomparison to when shutter 24 is closed so that the total amount oflight falling on the hologram during this dark mode is negligible.

In the flood light mode, reference beam shutter 26 is closedcontinuously and object beam shutter 24 is open continuously. Dataacquisition device 36 measures the diffracted light level atphotodetector 34 at regular intervals. In this mode, some auxiliarylight source can be used to flood the hologram, in which casephotodetector 34 must be shielded from this auxiliary light in order togive a true reading of the diffracted laser light.

The duration of each mode of operation can be a predetermined intervalor the duration can be ended when the measured diffraction efficiency ofthe hologram reaches a predetermined level as measured by photodetector34 with data acquisition device 36.

As will be appreciated, this device utilizes the light of a single laserto expose a hologram and to simultaneously measure the formation of thehologram in the recording material. This invention is applicable to theuse of holographic recording materials that begin to form hologramsalmost immediately upon exposure and including but not limited tophotopolymers, thermoplastics, and photochromic materials. The inventionfurther allows the measure of the hologram diffraction efficiency duringthe processing steps that are necessary after exposure such as chemicaldeveloping. The exposure and processing can be controlled with feedbackfrom these measurements.

I claim:
 1. A system for producing a hologram comprising; a laser lightsource, beam splitter means for splitting the laser light source into anobject beam and a reference beam, mirror reflecting means for reflectingthe object beam and the reference beam onto a holographic recordingmedium, shutters mounted in the path of the object beam and thereference beam before they are directed onto the holographic recordingmedium, a photodetector mounted beyond the holographic recording mediumand in the path of the reference beam, and control means for controllingthe opening and closing of the shutters and for recording data from saidphotodetector to monitor the amount of developing of the holographicrecording medium.
 2. A system as set forth in claim 1, wherein saidcontrol means is a computer.
 3. A system as set forth in claim 1,wherein said control means includes a shutter control that is a computerand a data acquisition device that is a computer to enable the shuttersin the object beam path and the reference beam path to he opened andclosed in predetermined sequences to expose the holographic recordingmedium as desired.